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Final Report Summary - SPIDERMAN (Electronic Transport and Spin dynamics through SiGe self-assembled quantum dots)

SiGe nanostructures have emerged as a promising material for the realization of spin qubits. In the past several years intense research has been devoted in studying QDs defined in SiGe 2DEGs and P donors in Silicon. Spin relaxation times of a few seconds and coherence times of close to a second have been reported. On the other hand there are much less studies which have focused on holes and in particular confined in SiGe. In 2012, spin relaxation times in Ge/Si NW QDs were reported to be in the ms range and just last year the dephasing time was reported to be more than one order of magnitude larger than that of III-V materials. All these studies point out the importance of group-IV elements in the field of spin qubits.
In our group we study holes confined in Ge self-assembled nanostructures. Ge self-assembled nanostructures are created by means of lattice-mismatched heteroepitaxial growth when a material with a larger lattice constant (Ge) is deposited on a smaller lattice constant substrate (Si). In the so called Stranski-Krastanow (SK) growth mode, the elastic strain stored in the growing film is relaxed by the formation of three-dimensional (3D) islands on top of a thin, pseudomorphic wetting layer.
Islands of different sizes and geometries [hut clusters, pyramids, domes] can be created.
In 2010 the first realisation of single-hole transistors based on individual SiGe nanocrystals was reported. Transport spectroscopy revealed largely anisotropic hole g-factors. By changing the number of holes localized within the SiGe QDs a clear modulation of the g-factor was observed indicating that the g-factors are linked to the corresponding orbital wavefunctions. However, dual gate devices demonstrated that the g-factor of the same orbital wavefunction can be changed by more than 300% when changing the value of a perpendicular electric field while keeping a constant number of holes.
This result indicated that this material system might be interesting for performing spin manipulation by means of g-tensor modulation.
However in order to move towards the realization of spin qubits one needs to move away from single quantum dot devices; charge sensors and double quantum dots need to be realized. This was the main focus of the PI's group.
For achieving these two building blocks, we are working with two different type of structures: a) self-organized Ge nanostructures, i.e. Ge islands grown on prepatterned Si. This approach might allow us to nucleate two islands next to each other, into a double dot configuration. b) one dimensional Ge nanowires, so called hut-wires. A particularly interesting feature of the HWs is that they are solely oriented along [100] and [010], whereas the wire height and width remain constant below 2 and 20 nm, leading thus to very strong confinement.

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Friedrich Schäffler, (Head)
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